Fix issues arising from E notation pricing by explicitly casting to float

[sfstar]

Larger floats get returned as:

This causes the following error:

Currently we are experiencing this for the dutch region due to some hours pricing having 0.000000E-05 or 8.99999E-x notation.

[MrOzzOnline]

Same in Belgium:
File "/config/custom_components/entsoe/coordinator.py", line 85, in parse_hourprices
hourprices[hour] = self.calc_price(value=price, fake_dt=hour)
File "/config/custom_components/entsoe/coordinator.py", line 79, in calc_price
price = round(template_value * (1 + self.vat), 5)
TypeError: type str doesn't define round method

I see a few public websites failing ...